Conventionally, as a technique to trap particulates (carbonaceous soot, high-boiling point hydrocarbon components (SOF), and other substances) contained in exhaust gas discharged from a diesel engine, a technique has been known that an engine exhaust emission purifier including a particulate filter is equipped along an exhaust stream of a diesel engine. The particulate filter has a porous honeycomb structure made of ceramics or other material, and the particulate filter has a configuration in which inlets of respective flow paths defined in a lattice form are alternately sealed up while outlets of flow paths whose inlets are unsealed are sealed up, to discharge only exhaust as that has permeated through porous walls defining the respective flow paths to a down-stream.
In such a particulate filter, trapped particulates are accumulated in the particulate filter to gradually increase exhaust resistance, and thus it is necessary to burn the accumulated particulates appropriately to regenerate the particulate filter. As a regeneration method of a particulate filter, a technique is known that an electrothermal heater is provided on an upper-stream side of an oxidation catalyst-loaded filter along an exhaust stream of a diesel engine, to raise the temperature of exhaust gas introduced to the oxidation catalyst-loaded filter by heating with the heater (see Patent document 1).
In the case where the diesel engine is put into a driving state such as idle driving, in which the exhaust flow volume is low during the forceful regeneration of the particulate filter by raising the temperature of the particulate filter, there is a problem in that not much of the heat generated due to combustion of particulates is brought out of the particulate filter through exhaust emission, which causes an excessive rise in temperature inside the particular filter, resulting in thermal degradation of the particulate filter. In view of this, a technique is known that, in the case where a deceleration of a vehicle is detected when the temperature of the particular filter is higher than a reference temperature, when a state in which the temperature of the particular filter is higher than the reference temperature continues for equal to or longer than a predetermined period of time, then exhaust flow volume reduction restraining means is operated to restrain a reduction in flow volume of exhaust flowing into the particular filter, while when the continued period of time of the state in which the temperature of the particular filter is higher than the reference temperature is shorter than the reference time, then the exhaust flow volume reduction restraining means is prohibited from operating (see, for example, Patent document 2).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-280121.    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-118461.